RFC 6590

Redaction of Potentially Sensitive Data from Mail Abuse Reports

Internet Engineering Task Force (IETF) J. Falk, Ed.
Request for Comments: 6590 Return Path
Category: Standards Track M. Kucherawy, Ed.
ISSN: 2070-1721 Cloudmark
April 2012 Redaction of Potentially Sensitive Data from Mail Abuse Reports
Abstract
Email messages often contain information that might be considered
private or sensitive, per either regulation or social norms. When
such a message becomes the subject of a report intended to be shared
with other entities, the report generator may wish to redact or elide
the sensitive portions of the message. This memo suggests one method
for doing so effectively.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6590.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction ....................................................22. Key Words .......................................................33. Recommended Practice ............................................34. Transformation Mechanisms .......................................45. Security Considerations .........................................55.1. General ....................................................55.2. Digest Collisions ..........................................55.3. Information Not Redacted ...................................56. Privacy Considerations ..........................................67. References ......................................................67.1. Normative References .......................................67.2. Informative References .....................................6Appendix A. Example ................................................7Appendix B. Acknowledgements .......................................81. Introduction
The Abuse Reporting Format [ARF] defines a message format for sending
reports of abuse in the messaging infrastructure, with an eye toward
automating both the generation and consumption of those reports.
For privacy considerations, it might be the policy of a report
generator to anonymize, or obscure, portions of the report that might
identify an end user who caused the report to be generated. This has
come to be known in feedback loop parlance as "redaction". Precisely
how this is done is unspecified in [ARF], as it will generally be a
matter of local policy. That specification does admonish generators
against being too overzealous with this practice, as obscuring too
much data makes the report non-actionable.
Previous redaction practices, such as replacing local-parts of
addresses with a uniform string like "xxxxxxxx", frustrated any kind
of prioritizing or grouping of reports. This memo presents a
practice for conducting redaction in a manner that allows a report
receiver to detect that two reports were caused by the same end user,
without revealing the identity of that user. That is, the report
receiver can use the redacted string, such as an obscured email
address, to determine that two such unredacted strings were
identical; the reports originally contained the same address.

Generally, it is assumed that the recipient-identifying fields of a
message, when copied into a report, are to be obscured to protect the
identity of the end user who submitted the complaint about the
message. However, it is also presumed that other data will be left
intact, and those data could be correlated against log files or other
resources to determine the intended recipient of the original
message.
2. Key Words
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [KEYWORDS].
3. Recommended Practice
When redacting of reports is desired, in order to enable a report
receiver to correlate reports that might refer to a common but
anonymous source, the report generator SHOULD use the following
practice:
1. Select a transformation mechanism (see Section 4) that is
consistent (i.e., the same input string produces the same output
each time) and reasonably collision-resistant (i.e., two
different inputs are unlikely to produce the same output).
2. Identify string(s) (such as local-parts of email addresses) in a
message that need to be redacted. Call these strings the
"private data".
3. For each piece of private data, apply the selected transformation
mechanism.
4. If the output of the transformation can contain bytes that are
not printable ASCII, or if the output can include characters not
appropriate to replace the private data directly, encode the
output with the base64 algorithm as defined in Section 4 of
[BASE64], or some similar translation, to form a valid
replacement in the original context. For example, replacing a
local-part in an email address with transformation output
containing an "@" character (ASCII 0x40) or a space character
(ASCII 0x20) is not permitted by the specification for local-part
[SMTP], so the transformation output needs to be encoded as
described.

5. Replace each instance of private data with the corresponding
(possibly encoded) transformation when generating the report.
Note that the replaced text could also be in a context that has
constraints, such as length limits that need to be observed.
This has the effect of obscuring the data (in a potentially
irreversible way) while still allowing the report recipient to
observe that numerous reports are about one particular end user.
Such detection enables the receiver to prioritize its reactions based
on problems that appear to be focused on specific end users that may
be under attack.
4. Transformation Mechanisms
This memo does not specify a particular transformation mechanism as a
requirement. The interoperability that this memo seeks to provide is
enabled by the consistency of the transformation.
Dealing with the issue of the security of the transformation (i.e.,
frustrating attempts to reverse the transformation) is a matter of
local policy. A continuum of possible transformations exists, from
trivial ones such as rot13, CRC32, and base64, through strong
cryptographic encodings such as the Hashed Message Authentication
Code [HMAC] and even full encryption, or private transformations such
as mapping an email address to an internal customer number. An
operator wishing to perform report redaction needs to select a
consistent transformation that obscures the private data and is
resilient to attempts to extract the original data to the extent
required by local policy, keeping in mind that the environment in
which the transformation is operating is not a highly secure one.
See Section 5.3 for further details of this issue.
An implementation MAY choose any transformation that has a reasonably
low likelihood of collision.

5. Security Considerations
5.1. General
General security issues with respect to these reports are found
in [ARF].
5.2. Digest Collisions
Message digest collisions are a well-understood issue. Their
application here involves a report receiver improperly concluding
that two pieces of redacted information were originally the same when
in fact they are not. This can lead to a denial of service, where
the inadvertently improper application of complaint data causes
unjustified corrective action. Such cases are sufficiently unlikely
as to be of little concern.
5.3. Information Not Redacted
Although the identity of the user causing a report to be generated
can be obscured using this mechanism, other properties of a message
(such as the Message-ID field) that are not redacted could be used to
recover the original data by locating them in the message logs of the
originating system or via other data correlation techniques. It is
incumbent on the report generator to anticipate and redact or
otherwise obscure such data, or accept that such recovery is possible
even from the very simplest kinds of feedback.
It is for this reason that the normative portions of this memo do not
include stronger assertions about cryptography used in the
transformation. Given the ultimate recoverability of the redacted
information, the cryptographic strength of the transformation is not
a critical security measure.
The process of redacting a feedback report satisfies a privacy
requirement established by local policy, and is not meant to provide
strong security properties.
[FBL-BCP] and Section 8 of [ARF] discuss topics related to
establishment of bilateral agreements between report producers and
consumers. The issues raised here are also things to be considered
when establishing such agreements.

Appendix A. Example
Assume the following input message:
From: alice@example.com
To: bob@example.net
Subject: Make money fast!
Message-ID: <123456789@mailer.example.com>
Date: Thu, 17 Nov 2011 22:19:40 -0500
Want to make a lot of money really fast? Check it out!
http://www.example.com/scam/0xd0d0cafe
On receipt, bob@example.net reports this message as abusive through
whatever mechanism his mailbox provider has established. This causes
an [ARF] message to be generated. However, example.net wishes to
obscure Bob's email address lest it be relayed to the offending
agent, which could lead to more trouble for Bob.
Thus, example.net plans to redact the local-part of the recipient
address in the To: field. Local policy and security requirements
suggest that the algorithm known as "H" (a hash of a key concatenated
with the data to be obscured) using SHA1 is adequate. It has thus
selected a redaction key of "potatoes", and the private data in this
case is the string "bob". The concatenation of "potatoesbob" is
digested with SHA1 and then base64-encoded to the string
"rZ8cqXWGiKHzhz1MsFRGTysHia4=".
Therefore, when constructing the ARF message in response to Bob's
complaint, the following form of the received message is used in the
third part of the ARF report:
From: alice@example.com
To: rZ8cqXWGiKHzhz1MsFRGTysHia4=@example.net
Subject: Make money fast!
Message-ID: <123456789@mailer.example.com>
Date: Thu, 17 Nov 2011 22:19:40 -0500
Want to make a lot of money really fast? Check it out!
http://www.example.com/scam/0xd0d0cafe
Note, however, that it is possible that the redacted information can
be recovered by agents at example.com searching their logs for the
original envelope associated with the message, by correlating with
the Message-ID contents, which were not redacted here. It is
expected that feedback loops generating such reports involve senders
that have been vetted against such information leakage.